Method for measuring a medium that flows through a measuring tube

ABSTRACT

Method for measuring the flow rate of a medium flowing through a measuring tube and which is penetrated by a constant magnetic field (B) orthogonal to the flow direction and in which the electrical voltage (UO) building up in the medium is sensed by means of electrodes ( 3, 3 ′), insulated with respect to the medium and located on the wall of the measuring tube ( 2 ) in a plane orthogonal to the flow direction of the medium, in which the capacitance of at least one capacitive unit (C) formed by the electrodes ( 3, 3 ′) and the tube interior and the charge change brought about by the change to the at least one capacitive unit (C) is determined and from this the voltage (UO) is established.

[0001] The invention relates to a method for measuring the flow rate ofa medium flowing through a measuring tube and which is penetrated by aconstant magnetic field positioned orthogonally to the flow direction,in which the electrical voltage building up in the medium is sensed byelectrodes located in the plane of the magnetic field and externally onthe electrically insulating wall of the measuring tube.

[0002] It is known to establish the flow of a medium through a measuringtube by capacitive measurement of the electrical voltage building up inthe medium (Helmut Brockhaus, magnetoinductive flow measurement withcapacitive methods, in the journal tm—Technisches Messen, 64, 1997, 5,Oldenbourg Verlag, Munich).

[0003] For practical reasons there is a considerable need for ameasuring system combining the advantages of capacitive coupling outwith those of a permanent magnetic excitation, the latter being theminuscule power requirement for building up the magnetic field. Such asystem would also be particularly suitable for micromechanicalimplementation.

[0004] Rudimentary solutions have been proposed in DE 198 31 491 A1 andDE 100 49 781 A1. However, following extensive testing it has been foundthat both proposals require galvanic auxiliary electrodes. The galvanicpotentials associated therewith cannot be completely eliminated whenapplying or using time-constant magnetic fields without working withcomplicated electrochemical reference electrodes which are problematicalin practical use.

[0005] Therefore the problem of the invention is to provide a flowmeasurement method working with low energy costs.

[0006] According to the invention this problem is solved by the featuresgiven in claim 1. The subclaims provide advantageous developments of theinvention.

[0007] According to the invention, for the capacitive coupling out ofthe measuring signal UO proportional to the flow rate of the flowmedium, an electrically insulating measuring tube penetrated by a staticmagnetic field of induction B is equipped with coupling out electrodesapplied externally and with the tube interior in each case forming acapacitive unit. The capacitance of at least one of these capacitiveunits is variable by a control device. The charge change caused by thecapacitance change is inter alia dependent on the voltage induced in theinterior of the measuring tube by interaction of the speed of theflowing medium with the magnetic field of the induction. By means of ameasuring and evaluating device linked with the capacitive units and thecontrol device, conclusions can be drawn regarding this useful voltageUO and finally, using known methods, the flow rate of the flowingmedium. The tube walls form at least part of the dielectric.

[0008] In principle, the method according to the invention provides fora large number of solutions concerning the technical implementation ofthe capacitance change. In an exemplified variant the capacitance changeis brought about by mechanical measures. For this purpose the couplingout electrodes can e.g. be positioned in such a way that their spacingwith respect to the inner wall of the measuring tube is variable. Thespacing control can e.g. take place by magnetic influencing or bypiezoelectric systems.

[0009] However, rotary arrangements are also conceivable. In this case,e.g. through sectorwise differing dielectric or geometric conditions ona rotary circular disk, the effective coupling out capacitance ischanged when the particular sectors pass over the coupling out area.

[0010] A non-mechanical variant is constituted by electronic capacitancecontrol using semiconductor layers in the coupling out zone, in thatthere is a virtually wattless or nondissipative operation thereof atdifferent control voltages applied in the inverse direction using theprinciple known from the varactor diode, so that different capacitancevalues arise.

[0011] Particularly promising is the use of ferroelectric layers oflimited thickness as the dielectric in capacitors, whose permittivitycan be varied within wide limits by the application of a controlvoltage. Jointly with the use of a permanent magnet for producing themagnetic field, this solution is ideally adapted to the manufacturingpossibilities of micromechanical systems, because ferroelectrics withlayer thicknesses of a few micrometres are sufficient. This makes itpossible to implement flow sensors with very small dimensions, such asare of interest in many fields, e.g. in medicine and the chemicalindustry.

[0012] The method according to the invention offers numerouspossibilities with regards to the time variation of the capacitancechange. In addition to non-periodic switching operation it is possibleto have periodic operating modes, e.g. with sinusoidal or square-wave orpulse behaviour, the period time being easily adaptable to the measuringrequirements.

[0013] The method according to the invention also offers numerouspossibilities with regards to signal evaluation and details thereof arenaturally dependent on the nature of the technical implementation of thecapacitance change. On the one hand the charge change can be directlymeasured at the varied capacitance, in that the latter is directlyconnected to a change amplifier, such as is known from piezoelectricmeasurement technology. On the other, preferably in periodic operation,the current occurring as a result of the load change or the voltage dropcorresponding thereto can be evaluated at a resistor in a closedmeasuring circuit, optionally following suitable amplification.

[0014] Thus, in summarizing, the invention indicates the way to solvinga problem hither to considered insoluble of extending the principle ofmagnetoinductive flow sensors to the use of static magnetic fields andat the same time permitting the utilization of the known advantages ofcapacitive signal coupling out. Particular mention is made of the factthat with permanent magnets from the group of rare earths, it ispossible to produce extremely high magnetic fields with minimumconstructional volumes, such as cannot be achieved with electricallyexcited systems. As the induced useful signal is proportional to theinduction, this advantage directly aids the level of the measuringsignal. In this way it is possible to implement a magnetoinductive flowsensor, which can be operated with a minimum power requirement withmaximum sensitivity, so as to provide excellent bases formains-independent operation. Together with the known advantage ofcompletely smooth inner walls of the measuring tube in the absence ofmechanically moving parts within the measuring medium, it is thuspossible to fulfill all the requirements concerning modern sensor means.

[0015] A particular advantage of the principle according to theinvention is the possibility of microsystem implementation with hithertounknown minimal dimensions in the mm range and below. This opens broadpossibilities of use in medicine, environmental technology and industry.

[0016] The invention is explained hereinafter relative to a drawing,which is a diagrammatic cross-section through the measuring tube withthe externally located coupling out electrodes with the control deviceused for modifying the same and the measuring and evaluating deviceconnected thereto.

[0017] On an axis orthogonal to the magnetic field lines designated B inthe drawing are provided coupling out electrodes 3, 3′ positionedoutside the electrically insulating measuring tube 2, whereof the sizeof at least one can be modified by the control device 7 connectedthereto (indicated by a sloping double arrow). Together with the tubeinterior the coupling out electrodes form capacitive units, which areused for coupling out the voltage UO induced in the interior 1 of themeasuring tube 2 as a result of the interaction between the flow rate vand magnetic field B, in that the charge change occurring as aconsequence of a capacitance change of the capacitive units isestablished in a measuring and evaluating device 4 connected by means ofconnections 5, 6, whereby from the same the useful voltage UO isdetermined and finally, using known laws, the flow rate is establishedtherefrom.

[0018] The following explanations serve in exemplified manner only forderiving the relationships between a capacitance change from C to C+ΔCand the charge change caused by the same, as well as the time behaviourof the current i and voltage UR observed in the measuring and evaluatingunit 4, where the latter in the simplest conceivable case merelycomprises a series resistor R inserted in the connecting leads 5, 6 andUR stands for the voltage falling thereon.

[0019] Using as a basis the state of equilibrium characterized by thesame voltage of in each case UO/2 at the identical output capacitances C(3, 3′), then as a result of the sudden bilateral increase of C to C+ΔC,the charge

ΔQ=2ΔC UO/2=ΔC UO

[0020] flows in the measuring circuit, which in closed circuit acrossthe resistor R leads to the flow$i = {\frac{1}{RC}\Delta \quad {C \cdot {UO} \cdot ^{- \frac{1}{RC}}}}$

[0021] and therefore supplies the voltage drop$U_{R} = {\frac{\Delta \quad C}{C} \cdot {UO} \cdot ^{- \frac{1}{RC}}}$

[0022] to R.

[0023] In the case of a sinusoidal capacitance change with the angularfrequency ω the voltage drop$U_{R} = {{\frac{\Delta \quad C}{C} \cdot {UO} \cdot \sin}\quad \omega \quad t}$

[0024] occurs, for as long as the reciprocal of the time constant RC canbe assumed as large compared with the angular frequency ω.

[0025] This simple observation makes clear the mechanism of the couplingout of the useful voltage UO through the parametric change of thecoupling out capacitors. With possible relative capacitance changesΔC/C>1, it is even possible to implement an amplification without usingexternal amplifiers, even though this possibility always additionallyexists.

[0026] The method according to the invention provides a large number ofalternative control and evaluation possibilities and the precedingsimple observation merely serves to explain the fundamentalimplementation of the idea according to the invention.

1. Method for measuring the flow rate of a medium flowing through ameasuring tube and which is penetrated by a constant magnetic field (B)orthogonal to the flow direction and in which the electrical voltage(UO) building up in the medium is sensed by means of electrodes (3, 3′),insulated with respect to the medium and located on the wall of themeasuring tube (2) in a plane orthogonal to the flow direction of themedium, characterized in that the capacitance of at least one capacitiveunit (C) formed by the electrodes (3, 3′) and the tube interior and thecharge change brought about by the change to the at least one capacitiveunit (C) is determined and from this the voltage (UO) is established. 2.Method according to claim 1, characterized in that the change to thecapacitance of the at least one capacitive unit (C) takes place bymodifying the spacing of the electrode (3) from the tube interior. 3.Method according to claim 2, characterized in that the change to thespacing of the electrode (3) with respect to the tube interior isbrought about by mechanical action.
 4. Method according to claim 2,characterized in that the change to the spacing of the electrode (3)relative to the tube interior takes place by magnetic action.
 5. Methodaccording to claim 2, characterized in that the change to the spacing ofthe electrode (3) relative to the tube interior takes placepiezoelectrically.
 6. Method according to claim 1, characterized in thatthe change to the capacitance of the at least one capacitive unit (C)takes place by modifying the permittivity of a material introduced intothe space between the electrode (3) and the tube interior.
 7. Methodaccording to claim 6, characterized in that the material is asemiconductor material, whose permittivity is dependent on a voltageapplied thereto in the inverse direction.
 8. Method according to claim6, characterized in that the material is a ferroelectric material, whosepermittivity is dependent on a control voltage applied thereto. 9.Method according to one of the preceding claims, characterized in thatthe charging current flowing on modifying the capacitance of the atleast one capacitive unit (C) is determined as a measured value forestablishing the induced voltage (UO) and therefore the flow rate of themedium.